Catalyst ink compositions and methods for forming hydrogen pumping proton exchange membrane electrochemical cell

ABSTRACT

A membrane electrode assembly (MEA) includes an ionically-conductive proton exchange membrane, an anode contacting a first side of the membrane and a cathode contacting a second side of the membrane and including third catalyst particles and a cathode GDL. The anode includes an anode gas diffusion layer (GDL), a first anode catalyst layer containing first catalyst particles, a hydrophobic polymer bonding agent, and a first ionomer bonding agent that lacks functional chains on a molecular backbone, and a second anode catalyst layer containing second catalyst particles and a second ionomer bonding agent that includes functional chains on a molecular backbone.

FIELD

The present disclosure relates generally to catalyst ink compositionsand methods of depositing the ink compositions to form electrochemicalcells used for hydrogen recovery in a fuel cell system.

BACKGROUND

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide regenerative fuelcells, that also allow reversed operation, such that oxidized fuel canbe reduced back to unoxidized fuel using electrical energy as an input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, and 3A are schematic diagrams of fuel cell systems accordingto various embodiments of the present disclosure.

FIG. 3B is a sectional perspective view of a central column of the fuelcell system of FIG. 3A.

FIG. 4 is a schematic side cross-sectional perspective view of a PEMelectrochemical cell, according to various embodiments of the presentdisclosure.

FIG. 5 is a schematic side cross-sectional view of a pump separatorincluding an electrochemical cell stack, according to variousembodiments of the present disclosure.

SUMMARY

According to various embodiments of the present disclosure, a method offorming membrane electrode assembly (MEA) comprises dispensing a firstanode ink comprising first catalyst particles, a hydrophobic polymerbonding agent and an ionomer bonding agent, dispersed in a firstcarrier; heat-treating the first anode ink to form a first anodecatalyst layer of an anode; dispensing a second anode ink on the firstanode catalyst layer, the second anode ink comprising second catalystparticles and an ionomer bonding agent, dispersed in a second carrier;heat-treating the second anode ink to form a second anode catalyst layerof the anode; dispensing a cathode ink; and heat-treating the cathodeink to form a cathode layer.

According to various embodiments of the present disclosure, an anode inkfor forming an anode layer of a carbon monoxide (CO) tolerant membraneelectrode assembly (MEA) comprises: catalyst particles comprisingplatinum or a platinum alloy; an ionomer binding agent; a hydrophobicbinding agent; at least two solvents selected from glycerol, ethyleneglycol, propylene glycol, N-methyl-2-pyrrolidone (NMP), isopropylalcohol (IPA), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK),acetone, ethyl lactate, diglyme, propylene glycol monomethyl etheracetate (PGMEA), butyl acetate, or heptanol; and water.

According to various embodiments of the present disclosure a membraneelectrode assembly (MEA) includes an ionically-conductive protonexchange membrane, an anode contacting a first side of the membrane anda cathode contacting a second side of the membrane and including thirdcatalyst particles and a cathode GDL. The anode includes an anode gasdiffusion layer (GDL), a first anode catalyst layer containing firstcatalyst particles, a hydrophobic polymer bonding agent, and a firstionomer bonding agent that lacks functional chains on a molecularbackbone, and a second anode catalyst layer containing second catalystparticles and a second ionomer bonding agent that includes functionalchains on a molecular backbone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be understood that when an element or layer is referred to asbeing “on” or “connected to” another element or layer, it can bedirectly on or directly connected to the other element or layer, orintervening elements or layers may be present. In contrast, when anelement is referred to as being “directly on” or “directly connected to”another element or layer, there are no intervening elements or layerspresent. It will be understood that for the purposes of this disclosure,“at least one of X, Y, and Z” can be construed as X only, Y only, Zonly, or any combination of two or more items X, Y, and Z (e.g., XYZ,XYY, YZ, ZZ).

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range is encompassed within the invention. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges is also encompassed within the invention, subject to anyspecifically excluded limit in the stated range. Where the stated rangeincludes one or both of the limits, ranges excluding either or both ofthose included limits are also included in the invention. It will alsobe understood that the term “about” may refer to a minor measurementerrors of, for example, 5 to 10%. In addition, weight percentages (wt %)and atomic percentages (at %) as used herein respectively refer to apercent of total weight or a percent of a total number of atoms of acorresponding composition.

Words such as “thereafter,” “then,” “next,” etc. are not necessarilyintended to limit the order of the steps; these words may be used toguide the reader through the description of the methods. Further, anyreference to claim elements in the singular, for example, using thearticles “a,” “an” or “the” is not to be construed as limiting theelement to the singular.

The first and second embodiments of the invention illustrate how theelectrochemical pump separator is used together with a fuel cell system,such as a solid oxide fuel cell system. It should be noted that otherfuel cell systems may also be used.

In the system of the first embodiment, a fuel humidifier is used tohumidify the fuel inlet stream provided to the fuel cell stack. In thesystem of the second embodiment, the fuel humidifier may be omitted. Aportion of the fuel cell stack fuel exhaust stream is directly recycledinto the fuel inlet stream to humidify the fuel inlet steam. Anotherportion of the fuel cell stack fuel exhaust stream is provided to theseparator, and the separated hydrogen is then provided to the fuel inletstream.

FIG. 1 is a schematic showing a fuel cell system 100 according to thefirst embodiment of the present disclosure. The system 100 contains afuel cell stack 101, such as a solid oxide fuel cell stack (illustratedschematically to show one solid oxide fuel cell of the stack containinga ceramic electrolyte, such as yttria stabilized zirconia (YSZ), ananode electrode, such as a nickel-YSZ cermet, and a cathode electrode,such as lanthanum strontium manganite).

The system 100 also contains an electrochemical pump separator 150 thatelectrochemically separates hydrogen from the fuel exhaust stream. Thepump separator 150 may comprise any suitable proton exchange membranedevice comprising a polymer electrolyte. Preferably, the pump separator150 comprises a stack of carbon monoxide tolerant electrochemical cells,such as a stack of high-temperature, low-hydration ion exchange membranecells. These cells generally operate in a temperature range of above100° C. to about 200° C. Thus, the heat exchangers in the system 100preferably keep the fuel exhaust stream at a temperature of about 120°C. to about 200° C., such as about 160° C. to about 190° C.

The system 100 also contains a fuel exhaust conduit 153 that operatively(i.e., fluidly) connects a fuel exhaust outlet 103 of the fuel cellstack 101 to an anode inlet 151 of the pump separator 150. The system100 also contains a product conduit 157 that operatively (i.e., fluidly)connects a cathode outlet 158 of the pump separator 150 to a fuel inletconduit 111 that operatively (i.e., fluidly) connects a fuel inlet 105of the stack 101 to an external fuel source. The system 100 alsocontains a separator exhaust conduit 159 that operatively (i.e.,fluidly) connects an anode outlet 152 of the pump separator 150 to ananode tail gas oxidizer (ATO) 140 or to an atmospheric vent.

The system 100 further includes a fuel humidifier 119 operativelyconnected to the fuel inlet conduit 111 and the separator exhaustconduit 159. In operation, the fuel humidifier 119 humidifies fuel infuel conduit 111, which includes recycled hydrogen, using water vaporcontained the separator exhaust output to the separator exhaust conduit159. The fuel humidifier 119 may comprise a polymeric membranehumidifier, such as a Nafion® membrane humidifier, an enthalpy wheel ora plurality of water adsorbent beds, as described for example in U.S.Pat. No. 6,106,964 and in U.S. application Ser. No. 10/368,425, bothincorporated herein by reference in their entirety. For example, onesuitable type of humidifier comprises a water vapor and enthalpytransfer Nafion® based, water permeable membrane available from PermaPure LLC. The fuel humidifier 119 passively transfers water vapor andenthalpy from the fuel exhaust stream into the fuel inlet stream toprovide a 2 to 2.5 steam to carbon ratio in the fuel inlet stream. Thetemperature of the fuel in the fuel inlet conduit 111 may be raised toabout 80 to about 90 degrees Celsius, by the fuel humidifier 119.

The system 100 also contains a recuperative heat exchanger 121 (e.g.,anode recuperator) operatively connected to the fuel inlet conduit 111and the fuel exhaust conduit 153. The heat exchanger 121 heats the fuelin the fuel inlet conduit 111 using heat extracted from the fuel exhaustin the fuel exhaust conduit 103. The heat exchanger 121 helps to raisethe temperature of the incoming fuel and reduces the temperature of thefuel exhaust, so that it may be further cooled in the condenser and suchthat it does not damage the fuel humidifier 119.

If the fuel cells are external fuel reformation type cells, then thesystem 100 contains a fuel reformer 123. The reformer 123 reforms ahydrocarbon fuel inlet stream into hydrogen and carbon monoxidecontaining fuel stream which is then provided into the stack 101. Thereformer 123 may be heated radiatively, convectively, and/orconductively by the heat generated in the fuel cell stack 101 and/or bythe heat generated in an optional ATO 140, as described in U.S. Pat. No.7,422,810, filed Dec. 2, 2004, incorporated herein by reference in itsentirety. Alternatively, the external reformer 123 may be omitted if thestack 101 contains cells of the internal reforming type, wherereformation occurs primarily within the fuel cells of the stack.

The system 100 also includes an air inlet conduit 130 fluidly connectedto an air inlet 107 of the stack 101. Optionally, the system 100includes an air preheater heat exchanger 125 operatively connected tothe air inlet conduit 130 and configured to preheat the air in the airinlet conduit 130 using heat extracted from the fuel exhaust in the fuelexhaust conduit 153. If desired, this heat exchanger 125 may be omitted.

The system 100 also includes an air exhaust conduit 132 fluidlyconnecting an air exhaust outlet 109 of the stack 101 to the ATO 140.The system 100 preferably contains an air heat exchanger 127 operativelyconnected to the air inlet conduit 130 and the air exhaust conduit 132.This heat exchanger 127 further heats the air in the air inlet conduit130 using heat extracted from the fuel cell stack air exhaust (i.e.,oxidizer or cathode exhaust) in the air exhaust conduit 132. If thepreheater heat exchanger 125 is omitted, then the air is provideddirectly into the heat exchanger 127 by a blower or other air intakedevice.

The system 100 also optionally includes an optional hydrogen cooler heatexchanger 129 operatively connected to the product conduit 157 and theair inlet conduit 130. The heat exchanger 129 extracts heat from theseparated hydrogen output from the pump separator 150, using air flowingthrough the air inlet conduit 130.

The system 100 may also contain an optional water-gas shift (WGS)reactor 128 operatively connected to the fuel exhaust conduit 153. TheWGS reactor 128 may be any suitable device that converts at least aportion of the water in the fuel exhaust into free hydrogen (H₂). Forexample, the WGS reactor 128 may comprise a tube or conduit containing acatalyst that converts some or all of the carbon monoxide and watervapor in the fuel exhaust stream into carbon dioxide and hydrogen. Thus,the WGS reactor 128 increases the amount of hydrogen in the fuelexhaust. The catalyst may be any suitable catalyst, such as an ironoxide or a chromium-promoted iron oxide catalyst. The WGS reactor 128may be operatively connected to the fuel exhaust conduit 153, betweenthe fuel heat exchanger 121 and the air preheater heat exchanger 125.

The system 100 may operate as follows. A fuel inlet stream (alsoreferred to as “fuel” or “fuel stream”) is provided to the fuel cellstack 101 through fuel inlet conduit 111. The fuel may comprise anysuitable hydrocarbon fuel, including but not limited to methane, naturalgas which contains methane with hydrogen and other gases, propane orother biogas, or a mixture of a carbon fuel, such as carbon monoxide,oxygenated carbon containing gas, such as methanol, or other carboncontaining gas with a hydrogen containing gas, such as water vapor, H₂gas or their mixtures. For example, the mixture may comprise syngasderived from coal or natural gas reformation.

As the fuel stream passes through the humidifier 119, the fuel stream ishumidified. The humidified fuel then passes through the fuel heatexchanger 121 where the humidified fuel is heated by the fuel cell stackfuel exhaust. The heated and humidified fuel is then provided into thefuel reformer 123, which is preferably an external reformer. Forexample, the fuel reformer 123 may comprise a reformer described in U.S.Pat. No. 7,422,810, filed on Dec. 2, 2004, incorporated herein byreference in its entirety.

The fuel reformer 123 may be any suitable device that is capable ofpartially or wholly reforming a hydrocarbon fuel to form acarbon-containing and free-hydrogen-containing fuel. For example, thefuel reformer 123 may be any suitable device which can reform ahydrocarbon gas into a gas mixture of free hydrogen and a carboncontaining gas. For example, the fuel reformer 123 may comprise acatalyst coated passage where a humidified biogas, such as natural gas,is reformed via a steam-methane reformation reaction to form freehydrogen, carbon monoxide, carbon dioxide, water vapor and optionally aresidual amount of unreformed biogas. The free hydrogen and carbonmonoxide are then provided into the fuel (i.e., anode) inlet 105 of thefuel cell stack 101. Thus, with respect to a fuel flow direction in thefuel inlet conduit 111, the humidifier 119 is located upstream of theheat exchanger 121, which is located upstream of the reformer 123, whichis located upstream of the stack 101.

The air or other oxygen containing gas (i.e., oxidizer) (also referredto as an “air inlet stream” or “air stream”) that is provided to thestack 101 through the air inlet conduit 130 is heated by the air heatexchanger 127, using the cathode exhaust in the air exhaust conduit 132.If desired, the air in the air inlet conduit 130 may also pass throughthe hydrogen cooler heat exchanger 129 and/or through the air preheaterheat exchanger 125, to further increase the temperature of the airstream, before providing the air into the stack 101.

During operation, the stack 101 generates electricity using the providedfuel and air, and generates the fuel exhaust and the air exhaust. Thefuel exhaust may contain hydrogen, water vapor, carbon monoxide, carbondioxide, some unreacted hydrocarbon fuel such as methane, and otherreaction by-products and impurities. The fuel exhaust may include about25% of the fuel provided to the stack 101.

The fuel exhaust is output from the fuel exhaust outlet 103 and providedto the pump separator 150 by the fuel exhaust conduit 153. The pumpseparator 150 electrochemically separates at least a portion of thehydrogen (H₂) contained in the fuel exhaust. The separated hydrogen isoutput from the cathode outlet 158 and provided to the fuel inletconduit 111 by the product conduit 157 where the hydrogen and is mixedwith incoming fresh fuel. Preferably, the hydrogen is provided to thefuel inlet conduit 111 upstream of the humidifier 119.

The fuel exhaust stream in the fuel exhaust conduit 153 is firstprovided into the heat exchanger 121, where its temperature is lowered,preferably to less than 200° C., while the temperature of the incomingfuel is raised. If the WGS reactor 128 and the air preheater heatexchanger 125 are present, then the fuel exhaust is provided through theWGS reactor 128 to convert at least a portion of the water vapor and amajority of the residual carbon monoxide into carbon dioxide andhydrogen. The temperature of the fuel exhaust is then further reducedwhile passing through the heat exchanger 125, by transferring heat tothe air in the air inlet conduit 130. The temperature of the fuelexhaust may be reduced to from about 90 to 110° C., for example.

The fuel exhaust is then provided to the anode inlet 151 of the pumpseparator 150 via conduit 153. The pump separator 150 may be configuredto separate a majority of the hydrogen from the fuel exhaust, such asabout 85% of the hydrogen in the fuel exhaust stream. Water vapor,carbon dioxide, carbon monoxide and remaining hydrocarbon gas remainingin the fuel exhaust may be provided to the humidifier 119 by conduit159.

In the fuel humidifier 119, a portion of the water vapor in the fuelexhaust is transferred to the fuel in the fuel inlet conduit 111 tohumidify the fuel. The fuel may be humidified to 80° C. to 90° C. dewpoint. The remainder of the fuel exhaust stream is then provided intothe ATO 140 along with the air (i.e., cathode) exhaust from the stack101, where the gasses are oxidized to provide low quality heat. Theexhaust from the ATO may be provided to an ATO exhaust conduit 161. Heatfrom the ATO 140 may be used to heat the reformer 123, it may beprovided to other parts of the system 100, or may be provided to devicesoutside the system 100, such as a building heating system.

The hydrogen separated by the pump separator 150 is output from theoutlet 158 and provided by the product conduit 157 to the fuel inletconduit 111, where it is mixed with incoming fuel. If desired, prior tobeing provided to the fuel inlet conduit 111, the hydrogen maybe cooledin heat exchanger 129, where the hydrogen stream exchanges heat with airin the air inlet conduit 130. The temperature of the hydrogen is loweredin the heat exchanger 129 before being provided into the fuel inletconduit 111. Thus, the hydrocarbon fuel is mixed with a low dew point,near ambient temperature, recycled hydrogen recovered from the anodeexhaust gas with the electrochemical hydrogen pump separator 150.

Thus, with respect to the flow direction of the fuel exhaust, the heatexchanger 121 is located upstream of the WGS reactor 128, which islocated upstream of the heat exchanger 125, which is located upstream ofthe pump separator 150, which is located upstream of the humidifier 119and the fuel inlet conduit 111.

FIG. 2 is a schematic showing a fuel cell system 200 according to thesecond embodiment of the present disclosure. The system 200 is similarto system 100 and contains a number of components in common. Thosecomponents which are common to both systems 100 and 200 are numberedwith the same numbers in FIGS. 1 and 2 and will not be describedfurther.

One difference between systems 100 and 200 is that system 200preferably, but not necessarily lacks, the humidifier 119. Instead, aportion of the water vapor containing stack fuel exhaust stream isdirectly recycled into the stack fuel inlet stream. The water vapor inthe fuel exhaust stream is sufficient to humidify the fuel inlet stream.

The system 200 may contain a fuel exhaust splitter 201, a recyclingconduit 203, a blower or compressor 205, and a mixer 207. The splitter201 may be a computer or operator controlled multi-way valve, forexample a three-way valve, or another fluid splitting device. Thesplitter 201 may be operatively connected to the fuel exhaust conduit153 and the recycling conduit 203. In particular, the splitter 201 maybe configured to selectively divert all or a portion of the fuel exhaustin the fuel exhaust conduit 153 to the recycling conduit 203.

The mixer 207 may be operatively connected to the fuel inlet conduit111, the recycling conduit 203, and the product conduit 157. Therecycling conduit 203 may fluidly connect the splitter 201 to the mixer207. The mixer 207 may be configured to mix fresh fuel with fuel exhaustprovided by the recycling conduit 203 and/or hydrogen provided by theproduct conduit 157.

The blower or compressor 205 may be operatively connected to therecycling conduit 203. The blower or compressor 205 may be configured tomove the fuel exhaust through the recycling conduit 203 to the mixer207. In operation, the blower or compressor 205 controllably provides adesired amount of the fuel exhaust to the fuel inlet conduit 111, viathe mixer 207.

The method of operating the system 200 is similar to the method ofoperating the system 100. One difference is that the fuel exhaust isseparated into at least two streams by the splitter 201. The first fuelexhaust stream is recycled to the fuel inlet stream, while the secondstream is directed into the pump separator 150 where at least a portionof hydrogen contained in the second fuel exhaust stream iselectrochemically separated from the second fuel exhaust stream. Thehydrogen separated from the second fuel exhaust stream is then providedinto the fuel inlet conduit 111 by the product conduit 157. For example,between 50% and 70%, such as about 60% of the fuel exhaust may beprovided to the blower or compressor 205, while the remainder may beprovided toward the pump separator 150.

Preferably, the fuel exhaust first flows through the heat exchangers 121and 125, and the WGS reactor 128, before being provided into thesplitter 201. The fuel exhaust may be cooled to about 200° C. or less,such as to about 120° C. to about 180° C., in the heat exchanger 125,and prior to being provided into the splitter 201 where it is dividedinto two streams. This allows the use of a low temperature blower 205 tocontrollably recycle a desired amount of the fuel exhaust stream intothe fuel inlet conduit 111, since such a blower may be adapted to move agas stream that has a temperature of about 200° C. or less.

The blower or compressor 205 may be computer or operator controlled andmay vary the amount of the fuel exhaust stream being provided into thefuel inlet stream depending on the conditions described below. In someembodiments, the system 200 may optionally include a selector valve 210operatively connected to the product conduit 157. The selector valve 210may be fluidly connected to an auxiliary device 212, such as a hydrogenstorage device a hydrogen using device, such as a PEM (i.e., protonexchange membrane, also known as polymer electrolyte membrane) fuel cellin a vehicle or another hydrogen using device or to a hydrogen storagevessel. The selector valve 210 may be configured to divert a selectedamount of the hydrogen in the product conduit 157 to the auxiliarydevice 212. For example, all or a portion of the hydrogen may beprovided to either the auxiliary device 212 or the mixer 207, or thehydrogen may be alternately provided to the mixer 207 and the auxiliarydevice 212.

The blower or compressor 205 and the optional selector valve 210 may beoperated by a computer or an operator to controllably vary the gas flowbased on one or more of the following conditions: i) detected orobserved conditions of the system 100 (i.e., changes in the systemoperating conditions requiring a change in the amount of hydrogen in thefuel inlet stream); ii) previous calculations provided into the computeror conditions known to the operator which require a temporal adjustmentof the hydrogen in the fuel inlet stream; iii) desired future changes,presently occurring changes or recent past changes in the operatingparameters of the stack 101, such as changes in the electricity demandby the users of electricity generated by the stack, changes in price forelectricity or hydrocarbon fuel compared to the price of hydrogen, etc.,and/or iv) changes in the demand for hydrogen by the hydrogen user, suchas the hydrogen using device, changes in price of hydrogen orhydrocarbon fuel compared to the price of electricity, etc.

It is believed that by recycling at least a portion of the hydrogenseparated from the fuel exhaust (i.e., tail) gas into the fuel inletconduit 111, a high efficiency operation of the fuel cell system isobtained. Furthermore, the overall fuel utilization is increased. Theelectrical efficiency (i.e., AC electrical efficiency) can range betweenabout 50% and about 60%, such as between about 54% and about 60% for themethods of the first and second embodiments when the per pass fuelutilization rate is about 75% (i.e., about 75% of the fuel is utilizedduring each pass through the stack). An effective fuel utilization ofabout 94% to about 95% is obtained when the per pass utilization isabout 75%, and about 85% of the fuel exhaust gas hydrogen is recycledback to the fuel cell stack by the pump separator 150. Even higherefficiency may be obtained by increasing the per pass fuel utilizationrate above 75%, such as about 76-80%. At steady-state, the methods ofthe first and second embodiments eliminate the need for generating steamwhen steam methane reformation is used to create the feed gas to thefuel cell. The fuel exhaust stream contains enough water vapor tohumidify the fuel inlet stream to the stack at steam to carbon ratios of2 to 2.5. The increase in net fuel utilization and the removal of heatrequirement to generate steam increases the overall electricalefficiency. In contrast, without recycling hydrogen, the AC electricalefficiency is about 45% for a fuel utilization rate within the stack ofabout 75% to 80%.

FIG. 3A is a schematic diagram of a fuel cell system 300 according tothe third embodiment of the present disclosure. The system 300 mayinclude a number of components similar to the components previouslydescribed with respect to the systems 100 and 200 of the first andsecond embodiments, which may be numbered with the same numbers as inFIGS. 1 and 2, and will not be described in detail.

FIG. 3B is a sectional perspective view of a central column 301 of thefuel cell system 300 according to the third embodiment of the presentdisclosure. Alternatively, the central column 301 may be included ineither of the systems 100, 200 of the first and second embodiments.Accordingly, the central column 301 may include a number of componentssimilar to the components previously described with respect to thesystems 100, 200, which may be numbered with the same numbers as inFIGS. 1 and 2, and will not be described in detail.

In addition to the components that were described with respect to thefirst and second embodiments, in the system 300, the column 301 may bedisposed inside a hot box 302 and a catalytic partial oxidation (CPOx)reactor 170, a CPOx blower 180 (e.g., air blower), a system blower 182(e.g., main air blower), the anode recycle blower 205, and the mixer 207may be disposed outside of the hotbox 302. However, the presentdisclosure is not limited to any particular location for each of thecomponents with respect to the hotbox 302.

Referring to FIG. 3A, the CPOx reactor 170 receives the fuel inletstream from a fuel inlet. The fuel inlet may be a utility gas lineincluding a valve to control an amount of fuel provided to the CPOxreactor 170. The CPOx blower 180 may provide air to the CPOx reactor 170during system 300 start-up, and then turned off during steady-stateoperating mode when the fuel cell stacks 101 reach a steady-stateoperating temperature above 700° C., such as 750 to 900° C. The fuel inthe steady state and/or a mixture of fuel and air during start-up may beprovided to the mixer 207 by the fuel inlet conduit 111.

The main air blower 182 may be configured to provide an air stream(e.g., air inlet stream) to the air preheater heat exchanger 125 throughair inlet conduit 130. The ATO exhaust stream flows from the ATO 140 tothe air heat exchanger (e.g., cathode recuperator) 127 through the ATOexhaust conduit 161. Exhaust flows from the air heat exchanger 127 tothe steam generator 160 through the ATO exhaust conduit 161. Exhaustflows from the steam generator 160 and out of the hotbox 302 through theATO exhaust conduit 161.

Water flows from a water source 190, such as a water tank or a waterpipe, to the steam generator 160 through water conduit 163. The steamgenerator 160 converts the water into steam using heat from the ATOexhaust provided by the ATO exhaust conduit 161. Steam is provided fromthe steam generator 160 to the mixer 207 through the water conduit 163.Alternatively, if desired, the steam may be provided directly into thefuel inlet stream and/or the anode exhaust stream may be provideddirectly into the fuel inlet stream followed by humidification of thecombined fuel streams.

The system 300 may further include a system controller 225 configured tocontrol various elements (e.g., blowers 182, 184 and 205 and the fuelcontrol valve) of the system 300. The controller 225 may include acentral processing unit configured to execute stored instructions. Forexample, the controller 225 may be configured to control fuel and/or airflow through the system 300, according to fuel composition data.

Referring to FIG. 3B, the central column may extend from a base 310 uponwhich one or more fuel cell stacks 101 may be disposed. The fuel inletconduit 111 and fuel exhaust conduit 153 may extend from the stacks 101,through the base 310, to the column 301.

The column 301 may include cylindrical outer and inner walls that atleast partially define an ATO 140. The fuel heat exchanger 121 may bedisposed around the reformer 123. An optional first additional WGSreactor 128A may be incorporated into the fuel heat exchanger 121 and/orthe reformer 123 by providing the WGS catalyst in the fuel heatexchanger 121 and/or the reformer 123. The ATO 140 may surround the fuelheat exchanger 121. The fuel cell stacks 101 may surround the ATO 140,and the air heat exchanger 127 (shown in FIG. 3A) may surround the fuelcell stacks 101 in the hot box 302.

The WGS reactor 128 is disposed above the fuel heat exchanger 121. Theair preheater heat exchanger 125 is disposed above the WGS reactor 128.An optional second additional WGS reactor 128B may be incorporated intothe air preheater heat exchanger 125 by providing the WGS catalyst inthe air preheater heat exchanger 125.

The fuel exhaust conduit 153 may fluidly connect the fuel cell stacks101, the fuel heat exchanger 121, the WGS reactor 128, and the airpreheater 125. Accordingly, a fuel exhaust stream output from the stacks101 may flow into the bottom of the column 301, along the outer surfaceof the fuel heat exchanger 121, and then may flow inside of the WGSreactor 128 and the air preheater heat exchanger 125, before exiting thetop of the column 301.

The fuel inlet conduit 111 may fluidly connect the stacks 101, the fuelheat exchanger 121, and the fuel reformer 123 to the fuel inlet.Accordingly, a fuel inlet stream may flow into the top of the column301, and then be provided to the fuel heat exchanger 121 and the fuelreformer 123, before exiting the bottom of the column 301 and beingprovided to the stacks 101.

For example, the fuel heat exchanger 121 may include a corrugatedseparator configured to separate the fuel inlet stream from the fuelexhaust stream. In some embodiments, a surface of the fuel heatexchanger 121 that contacts the fuel exhaust may be coated with a WGScatalyst, such that the fuel heat exchanger 121 may operate as acombined fuel heat exchanger 121 and WGS reactor 128A. In other words,the fuel heat exchanger 121 may operate to both transfer heat betweenthe fuel inlet and fuel exhaust streams, as well as convert water andcarbon monoxide in the fuel exhaust into free hydrogen and carbondioxide. As such, an additional volume in the column 301 may bededicated to WGS reactions, in order to increase WGS reactivity.

In some embodiments, the column 301 may include the splitter 201configured to divert a portion of the fuel exhaust stream to the ATO140. A remainder of the fuel exhaust stream may be cooled in the airpreheater heat exchanger 125 to temperature compatible with any furtherprocessing. The air preheater heat exchanger 125 conduits which carrythe fuel exhaust stream may optionally be coated in with a WGS catalystto allow the heat exchanger 125 to perform as an optional secondadditional WGS reactor 128B, if further reduction in carbon monoxidecontent and conversion of water to hydrogen is desired.

Another optional embodiment shown in FIG. 3A includes an electrochemicalpump separator 150 with an integrated third additional WGS reactorcatalyst. In one aspect of this optional embodiment, the anode catalystdescribed above is also a WGS catalyst. In another aspect of thisoptional embodiment, the WGS catalyst may be coated on the entire anodeof the electrochemical pump separator 150 or on a portion of the anodeof the electrochemical pump separator 150. In another aspect of thisoptional another embodiment, the WGS catalyst is located in the samehousing as the electrochemical pump separator 150 such that there is noair pre-heater 125 located between the electrochemical pump separator150 and the WGS catalyst. For example, the WGS catalyst may be coated ona surface of the anode chamber of the housing containing theelectrochemical pump separator 150, such that the fuel exhaust streampasses by the WGS reactor catalyst before reaching the anode of theelectrochemical pump separator 150. The WGS reactor catalyst maycomprise, PtRu, Cu, Cu—Zn, Cu—Zn—Al, Pt, Pt—Ni, Ni, Fe—Cr, or Fe—Cr—Cu.The WGS reactor catalyst may have an operating temperature of about200-450° C., such as about 200-250° C., which may be desirable formaximizing conversion of carbon monoxide and water into usable hydrogen(plus waste carbon dioxide).

Proton Exchange Membrane (PEM) Cells

Carbon monoxide (“CO”) is a poison for many hydrogen pump materials,substantially increasing the pumping voltage (power). Traditionally,binary catalysts, such as Pt—Ru, are employed for CO tolerance in protonexchange membrane (“PEM”, also known as polymer electrolyte membrane)fuel cells and exhibit suitable performance at CO concentrations of lessthan 100 ppm. However, the performance of such catalysts may besignificantly degraded at higher CO concentrations. In addition, an airbleed is generally necessary to ensure stable performance.

Fuel cell systems, such as the systems of FIGS. 1, 2, 3A, and 3B maygenerate a fuel cell fuel exhaust stream that contains a significantfraction of CO, such as reformate from the steam-reformation of naturalgas used as the fuel. For example, a fuel exhaust may have a COconcentration of above 100 ppm, such as 150 to 900 ppm.

Accordingly, various embodiments of the present disclosure provide a PEMcell that includes a membrane electrode assembly (MEA) that is capableof stably operating as a hydrogen pump, when supplied with ahydrogen-containing gas having CO concentration of above 100 ppm.

FIG. 4 is a cross-sectional view of a PEM electrochemical cell 400,according to various embodiments of the present disclosure. On or moreof the PEM cells 400 may be included in the pump separators 150 of FIGS.1, 2, and 3A.

Referring to FIG. 4, the PEM cell 400 may include a membrane electrodeassembly (MEA) 410, which may be disposed between flow field plates(e.g., bipolar plates) 402. The MEA 410 may include a polymerelectrolyte membrane 440 disposed between an anode 420 and a cathode430. In some embodiments the anode 420 may include an anode gasdiffusion layer (GDL) 426, and the cathode 430 may include a cathode GDL436. The anode GDL 426 may be configured to distribute ahydrogen-containing gas, such a fuel cell fuel exhaust stream from theSOFC stacks 101 shown in FIG. 3A, to the anode 420. The cathode GDL 436may be configured to collect hydrogen which diffused from the anode 420through the electrolyte membrane 440 to the cathode 430.

As discussed below in more detail, the anode 420 may include one or morelayers of catalyst particles that are deposited on (e.g., loaded onand/or coated on) the anode GDL 426, and the cathode 430 may include oneor more layers of catalyst particles that are deposited on (e.g., loadedon and/or coated on) the cathode GDL 436 to form respective gasdiffusion electrodes (GDE). Accordingly, the anode 420 may be referredto as a gas diffusion anode 420, and the cathode 430 may be may bereferred to as a gas diffusion cathode 430. Alternatively, the anodeand/or the cathode catalyst particle layers may be deposited on themembrane 440 instead of the anode GDL 426 and the cathode GDL 436,and/or deposited on both the membrane 440 and at least one of the anodeGDL 426 and the cathode GDL 436.

The flow field plates 402 may include an inlet flow field 404 configuredto provide the hydrogen-containing gas to the anode GDL 426, and anoutlet flow field 406 configured to receive hydrogen gas from thecathode GDL 436. In particular, the inlet flow field 404 and/or theoutlet flow field 406 may include flow channels configured to controlpressure and/or reactant concentrations at the membrane 440, inconjunction with the anode and cathode GDLs 426, 436.

The anode and cathode GDLs 426, 436 may be formed of materialsconfigured to reduce mass transfer loss, provide high electricalconductivity, and effectively manage water transport. For example, theanode and cathode GDLs 426, 436 may include a porous base material, suchas a porous carbon material, for example, carbon paper, carbon cloth,carbon felt, or a combination thereof, to provide electricalconductivity and effective mass transfer. The anode and/or cathode GDLs426, 436 may also include a hydrophobic material, such aspolytetrafluorethylene (PTFE). For example, the base material of theanode GDL 426 may be loaded with from about 5 weight percent (“wt %”) toabout 50 wt %, such as from about 10 wt % to about 30 wt % PTFE.

In some embodiments, anode and cathode GDLs 426, 436 may optionally bein the form of, or include, a microporous layer. In various embodiments,the anode and cathode GDLs 426, 436 may include multiple stacked layersto match the functionalities of the anode 420 and the cathode 430. Theanode and cathode GDLs 426, 436 may be formed from sheets, rolls, foils,or the like, according to the requirements of a manufacturing process.

In some embodiments, the anode 420 may include a microporous layer 428deposited on a surface of the anode GDL 426 that faces the membrane 440.The microporous layer 428 may include a hydrophobic material, such asPTFE, or a mixture of a hydrophobic material and an electricallyconductive material, such as carbon black. The microporous layer 428 maybe configured to improve anode water management. In some embodiments,the anode 420 may include the anode GDL 426, the microporous layer 428and a single anode layer (e.g., a single anode catalyst layer) 422 or424. In another embodiment, the cathode 430 may also include amicroporous layer in addition to the microporous layer 428 of the anodeor instead of the microporous layer 428 of the anode.

The membrane 440 may include any suitable proton conducting membrane,such as a proton exchange membrane. The membrane 440 may includeionomers, such as a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer having a chemical formula: C₇HF₁₃O₅S.C₂F₄ soldunder the brand name Nation®. Alternatively, the membrane 440 mayinclude a phosphoric acid membrane such as a polybenzimidazole(PBI)-based phosphoric acid membranes comprising poly-phosphoric acidand polybenzomidazole polymer), proton conducting oxides includephosphates such as LaPO₄, solid acids (such as cesium dihydrogenphosphate, CsH₂PO₄), and certain perovskite (ABO₃) materials such asperovskite type cerates, niobates, phosphates, gallates or zirconates,such as BaCeYO (BCO), BaZrYO (BZO), LaSrPO, BaCaNbO (BCN), LaCaNbO, orLaBaGaO (LBGO) those described in Chem. Soc. Rev., 2010, 39, 4370-4387,incorporated herein by reference in its entirety.

The PEM cell 400 may be operated in a hydrogen pumping mode (i.e., as anelectrolyzer), where an external voltage potential is applied betweenthe anode 420 and cathode 430, while the fuel exhaust is provided to theanode side may dissociate into protons and electrons. The electrons maybe externally routed to the cathode 430, and the protons may betransported (i.e., pumped) through the membrane 440 to the cathode 430,where the hydrogen ions recombine with electrons, resulting in hydrogenevolution.

The fuel cell exhaust provided to the cell 400 may have a COconcentration of above 100 ppm. Accordingly, the MEA 410 may beconfigured to tolerate such CO concentrations. In particular, the anode420 may be configured to tolerate long term exposure to COconcentrations of above 100 ppm, without the need to utilize an airbleed. However, if desired, an air bleed may be added to the fuelexhaust provided to the anode 420.

According to various embodiments, the anode 420 and/or cathode 430 mayinclude catalyst particles comprising one or more catalyst metals, suchas Pt, Ru, Ni, Rh, Mo, Co, Ir, Fe, and/or other d block metals includingpartially filled 3d orbitals. For example, the catalyst particles mayinclude elemental metals or alloys comprising Ag, Al, Au, Co, Cr, Cu,Fe, Ir, Mo, Ni, Pd, Pt, Ru, Rh, W, Zn, or any combination thereof. Forexample, the anode catalyst particles may include alloy catalyst such asPt—Ru, Cu—Zn, Cu—Zn—Al, Pt—Ni, Fe—Cr, or Fe—Cr—Cu.

The catalyst particles may each include a single catalyst metal, or analloy of catalyst metals. In some embodiments, the anode may includecatalyst particles that may include different catalyst metals anddifferent particle sizes. For example, the catalyst particles may rangefrom several nanometers to less than a micron in average particle size.In some embodiments, the catalyst particles may include a core-shellconfiguration, with a catalyst metal shell and a different (e.g., lessexpensive) metal or material as a core, in order to improve catalyticefficiency and reduce costs.

In some embodiments, the anode 420 may include Pt as a primary catalystfor hydrogen pumping (e.g., proton generation) and may include Ru, Ni,Rh, etc. as a secondary catalyst for promoting the oxidation of CO andthe suppression of the reverse water gas shift reaction of CO₂.

The anode 420 and/or cathode 430 may include one or more supportmaterials that may be loaded with the catalyst particles. Catalystsupport materials may be selected to provide for reduced catalystsloading, increased electrical conductivity, and better electrodeprinting characteristics. Common support materials may includecarbon-based materials, such as carbon black (e.g., Vulcan XC 72R carbonpowder, or a Ketjen Black powder), graphite powder, graphene, carbonnanotubes (CNTs), nano-horns, carbon fiber, carbon whiskers, or thelike. In other embodiments, catalyst support materials may includenon-carbon-based materials, such carbides, oxides, and nitrides ofmetals such as W, Ti, Mo, or the like.

In some embodiments, the anode and cathode 420, 430 may have a catalystparticle loading ranging from about 0.05 mg/cm² to about 3 mg/cm², suchas from about 0.1 mg/cm² to about 2 mg/cm². In particular, the anode 420may have a catalyst particle loading of from about 0.1 mg/cm², to about3 mg/cm², such as from about 0.1 mg/cm², to about 2 mg/cm². The cathode430 may have a catalyst particle loading ranging from about 0.05 mg/cm²,to about 1 mg/cm², such as from about 0.05 mg/cm², to about 0.5 mg/cm².

In various embodiments, the anode 420 and/or cathode 430 may include abonding agent. For example, the bonding agents may include a hydrophobicbonding agent such as PTFE and/or ionomer bonding agents such asNation®. The anode 420 and/or the cathode 430 may include from about 5wt % to about 50 wt %, such as from about 10 wt % to about 40 wt %, orfrom about 10 wt % to about 30 wt %, of a bonding agent or combinationof bonding agents.

According to various embodiments, the anode 420 may include a singlelayer or may be a multilayered anode including two, three, or morelayers. For example, as shown in FIG. 4, the anode 420 may have abilayer configuration including a first anode catalyst layer 422 (e.g.,first anode catalyst sub-layer) contacting the anode GDL 426 andcomprising first catalyst particles configured to preferentiallycatalyze the electrochemical oxidization of CO to form CO₂, and a secondanode catalyst layer 424 (e.g., second anode catalyst sub-layer)contacting the membrane 440 and comprising second catalyst particlesconfigured to preferentially catalyze the generation of hydrogen ions(i.e., protons) from H₂.

The first anode catalyst layer 422 may be bonded to the anode GDL 426,while the second anode catalyst layer 424 (e.g., second sub-layer) maybe bonded to the membrane 440 in a bilayer anode 420. If additionalanode layers are present in the anode 420, then the first anode catalystlayer 422 is generally located between the second anode catalyst layer424 and the anode GDL 426, while the second anode catalyst layer 424 isgenerally located between the first anode catalyst layer 422 and theelectrolyte membrane 440.

In some embodiments, the first anode catalyst layer 422 may have ahigher catalyst particle loading than the second anode catalyst layer424. In some embodiments, the loading amounts of the catalyst particlesin the first and second anode layers 422, 424 may vary in a thicknessdirection perpendicular to the membrane 440.

In various embodiments, the first anode catalyst layer 422 may includethe first catalyst particles and a hydrophobic bonding agent such asPTFE or the like. The hydrophobic bonding agent may be configured tocontrol the water content of the first anode catalyst layer 422. Forexample, the first anode catalyst layer 422 may include from about 5 wt% to about 50 wt %, such as from about 10 wt % to about 40 wt %, or fromabout 10 wt % to about 30 wt %, of the hydrophobic bonding agent such asPTFE.

The first catalyst particles may include catalyst metals or metal alloysas described above. For example, the first catalyst particles maycomprise a Pt—Ru alloy particles having a Pt:Ru atomic ratio rangingfrom about 60:40 to about 40:60, or about 50:50. In some embodiments,the first anode catalyst layer 422 may include the first catalystparticles, such as Pt—Ru alloy particles, at a loading ranging fromabout 0.3 mg/cm² to about 1.2 mg/cm², such as from about 0.8 mg/cm² toabout 1.2 mg/cm², from about 0.9 mg/cm² to about 1.1 mg/cm², or about 1mg/cm².

The first anode catalyst layer 422 may be configured to preferentiallyoxidize carbon monoxide via, for example, the water gas shift reaction.Accordingly, the first anode layer may be configured to reduce COpoisoning of the second anode catalyst layer 424.

The second anode catalyst layer 424 may include second catalystparticles having a composition different from the first catalystparticles and an ionomer bonding agent. The second catalyst particlesmay include catalyst metals or metal alloys as described above. Forexample, the second catalyst particles may comprise a Pt—Ru alloyparticles having a Pt:Ru atomic ratio ranging from about 55:45 to about70:30, or about 66:33. In one embodiment, if both the first and thesecond anode layers comprise the Pt—Ru alloy catalyst particles, thenthe Pt—Ru catalyst particles in the second anode catalyst layer 424 mayhave a higher Pt:Ru atomic ratio than the Pt—Ru catalyst particles inthe first anode catalyst layer 422. The second anode catalyst layer 424may include an ionomer bonding agent such as Nafion®. However, thepresent disclosure is not limited to any particular ionomer bondingagent. In various embodiments, the second anode catalyst layer 424 mayinclude the second catalyst particles, such as Pt—Ru alloy particles, ata loading ranging from about 0.3 mg/cm² to about 1.2 mg/cm², such asfrom about 0.5 mg/cm² to about 0.9 mg/cm², from about 0.6 mg/cm² toabout 0.8 mg/cm², or about 0.7 mg/cm².

In some alternative embodiments, the first and second anode layers 422,424 may include catalyst particles having similar Pt:Ru atomic ratios orthe same Pt:Ru atomic ratio. For example, the first and second anodelayers 422, 424 may include catalyst particles having a Pt:Ru atomicratio ranging from about 90:10 to about 10:90, such as from about 80:20to about 20:80, or from about 70:30 to about 30:70.

The cathode 430 may include a third catalyst layer 432 disposed on abetween the cathode GDL 436 and the membrane 440. The third catalystlayer 432 may include third catalyst particles and an ionomer bondingagent. The third catalyst particles may include catalyst metals or metalalloys as described above. For example, the third catalyst particles mayinclude Pt or a Pt alloy. The third catalyst layer 432 may also includea catalyst support comprising a carbon-based material or a metal oxide,nitride, or carbide material, as described above. In some embodimentsthe third catalyst layer 432 may include Pt catalyst particles supportedby a carbon-based material and bonded with an ionomer bonding agent,such as Nafion®. The third catalyst layer 432 may include the thirdcatalyst particles, such as Pt particles, at a loading ranging fromabout 0.025 mg/cm² to about 0.75 mg/cm², such as from about 0.05 mg/cm²to about 0.5 mg/cm².

When a voltage potential is applied between the cathode 430 and theanode 420, the second anode catalyst layer 424, the membrane 440, andthe cathode 430 may be configured to operate as a hydrogen pump. Inparticular, hydrogen ions generated at the second anode catalyst layer424 may be transported through the membrane 440 by a potentialdifference applied between the anode 420 and the cathode 430, and thehydrogen ions may then be recombined in the cathode 430 to generatehydrogen gas (H₂).

In other embodiments, the anode 420 may include a single layer structureincluding the first catalyst particles, the second catalyst particles,the ionomer, and PTFE. For example, the anode 420 may include themicroporous layer 428 and a single catalyst layer (422 or 424) disposedbetween the microporous layer 428 and the membrane 440. Relative amountsof the first catalyst particles and the second catalyst particles varyfrom 0:1 to 1:0, in a thickness direction of the anode 420, taken in adirection perpendicular to a plane of the membrane 440. For example, theanode 420 may have a relatively high concentration of the first catalystparticles adjacent to the anode GDL 426 and a relatively lowconcentration of the first catalyst particles adjacent to the membrane440, and the anode 420 may have a relatively high concentration of thesecond catalyst particles adjacent to the membrane 440 and a relativelylow concentration of the second catalyst particles adjacent to the anodeGDL 426. In other words, the relative amounts of Pt and Ru in the anode420 may vary in the thickness direction of the anode 420, such that theanode 420 has a lower Pt:Ru ratio adjacent to the anode GDL 426 and ahigher Pt:Ru ratio adjacent to the membrane 440. For example, the Pt:Ruratio may range from 90:10 to 10:90, in the thickness direction, in someembodiments.

In some embodiments, the relative amounts of the ionomer and PTFE mayvary from 0:1 to 1:0, in a thickness direction of the anode 420, takenin a direction perpendicular to a plane of the membrane 440. In someembodiments, the anode 420 may include a 50:50 weight ratio of ionomerand PTFE. The ionomer and PTFE may be bonded together as a dual bondingagent. The anode 420 may optionally include a micro porous layerincluding PTFE and/or an ionomer.

Electrode Ink Compositions and MEA Formation Methods

In various embodiments, at least one of the anode 420 and/or the cathode430 catalyst layers 422, 424 and 432, respectively, may be formed bydepositing corresponding anode and/or cathode inks on the respectiveanode and cathode GDLs 426, 436 to form respective GDEs. If themicroporous layer 428 is present, then the anode catalyst layer orlayers 422 and/or 424 may be deposited on the microporous layer 428.Alternatively, one or both of the inks may be deposited on the membrane440.

The microstructure of an anode or cathode electrode (e.g., catalystdispersion, porosity, and tortuosity) may be important in determiningthe functional performance of an MEA. In order to form a bi-layerelectrode having different bonding agents in each layer of the bi-layerelectrode, different electrode ink compositions may be used for eachlayer and/or different methods may be used by which an electrode ink isformed, applied, and/or processed after being applied.

According to various embodiments, the electrode inks may include asolids component dispersed in a liquid carrier. The solids component mayinclude catalyst particles, a bonding component (i.e., bonding agent),and optionally a catalyst support component. The carrier may include oneor more solvents and optionally one or more stabilizer components. Thesolvents may include organic solvents, inorganic solvents, water, or acombination thereof. The stabilizer component may include one or moresurfactants and/or one or more simple organic ligands.

The carrier may be formed of materials selected to provide stability tothe electrode ink. For example, the carrier may include one or moresolvents and/or stabilizers configured to suspend the solids componentas a colloidal suspension. In particular, the solvents and/orstabilizers may be chosen in view of stably suspending micron ornanometer size catalyst particles having a high surface area and a lowsolid loading amount, as well as a solubility of the bonding agents. Insome embodiments, the components of the carrier may be selected based ondesired catalyst loading amounts, desired ink wetting performance and/orbased on the desired electrical performance of an electrode formed fromthe electrode ink.

In various embodiments, the electrode inks may include the solidscomponent in an amount ranging from about 2 wt % to about 20 wt %, basedon a total weight percent of the electrode ink. The electrode ink mayinclude the carrier in an amount ranging from about 5 wt % to about 50wt %, based on a total weight percent of the electrode ink.

Suitable catalyst particles may include one or more catalyst metals,such as Pt, Ru, Ni, Rh, Mo, Co, Ir, Fe, and/or other d block metalsincluding partially filled 3d orbitals, as described above. For example,the catalyst particles may include elemental metals or alloys comprisingAg, Al, Au, Co, Cr, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Ru, Rh, W, Zn, or anycombination thereof. For example, the anode catalyst particles mayinclude alloy catalyst such as Pt—Ru, Pt—Co, Pt—Mn, Cu—Zn, Cu—Zn—Al,Pt—Ni, Fe—Cr, or Fe—Cr—Cu. In some embodiments, the catalyst particlesmay comprise a Pt—Ru alloy particles having a Pt:Ru atomic ratio rangingfrom about 70:30 to about, 30:70, about 60:40 to about 40:60, or about50:50.

The catalyst particles may each include a single catalyst metal, or analloy of catalyst metals. In some embodiments, the anode catalyst layersmay include catalyst particles that may include different catalystmetals and/or different particle sizes. For example, the catalystparticles may range from several nanometers to less than a micron, suchas 10 nm to 900 nm, in average particle size. In some embodiments, thecatalyst particles may include a core-shell configuration, with acatalyst metal shell and a different (e.g., less expensive) metal ormaterial as a core, in order to improve catalytic efficiency and reducecosts.

In various embodiments, an electrode ink may include the catalystparticles, such as Pt—Ru alloy particles, in an amount ranging fromabout 30 wt % to about 100 wt %, based on the total weight of the solidscomponent. In some embodiments, the catalyst particles may befunctionalized with organic ligands (e.g., may include a functionalizedsurface layer) in order to improve solubility in an electrode ink and/orink solvent.

The bonding component may include hydrophobic polymer bonding agents,such as PTFE or the like, and/or ionomer bonding agents, such as asulfonated tetrafluoroethylene based fluoropolymer-copolymer having achemical formula C₇HF₁₃O₅S.C₂F₄ sold under the brand name Nation®, orthe like. In some embodiments, the bonding component may represent fromabout 5 wt % to about 50 wt %, such as from about 10 wt % to about 40 wt%, or from about 10 wt % to about 30 wt % of a solids content of theelectrode ink.

The catalyst support component may include one or more support materialsconfigured to provide reduced catalysts loading, increased electricalconductivity, and better electrode printing characteristics. Commonsupport materials may include carbon-based materials, such as carbonblack (e.g., Vulcan XC 72R carbon powder, or a Ketjen Black powder),graphite powder, graphene, carbon nanotubes (CNTs), nano-horns, carbonfiber, carbon whiskers, or the like. In other embodiments, catalystsupport materials may include non-carbon-based materials, such carbides,oxides, and nitrides of metals such as W, Ti, Mo, or the like.

Solvent properties may impact the performance and/or manufacturabilityof an electrode formed from a corresponding electrode ink. For example,solvents may be selected based on a desired polarity, vapor pressure,dielectric constant, viscosity, stability, and/or boiling point thereof.Solvents may also be selected based on a desired ink wetting behaviorwhen the ink is applied to a GDL.

Suitable solvents may include organic solvents, such as alcohols,ethers, esters, ketones, acids, amines, glycols, glymes, glycerols,combinations thereof, of the like. For example, in some embodiments, anelectrode ink solvent may include glycerol, ethylene glycol, propyleneglycol, N-methyl-2-pyrrolidone (NMP), isopropyl alcohol (IPA), methylethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetone, ethyllactate, diglyme, propylene glycol monomethyl ether acetate (PGMEA),butyl acetate, heptanol, combinations thereof, or the like. The organicsolvent or combination thereof may be used alone, or may be mixed withother solvents and/or water at a ratio that may be selected to attaindesirable dispersion and/or surface wetting characteristics. In variousembodiments, an electrode ink may include one or more of the solvents inan amount ranging from about 5 wt % to about 50 wt %, based on the totalweight of the electrode ink.

The stabilizer may be selected based on catalyst particle size, surfacearea, loading amount, and/or based on the nature of solvents included inthe electrode ink. In some embodiments, electrode ink stabilizer mayhave a burn-off temperature that is less than or equal to a sinteringtemperature of an electrode layer formed by the electrode ink. Sinteringtemperatures may depend upon the type of catalyst and the thermalstability of an associated bonding component. For example, thehydrophobic bonding agents, such as PTFE may have a higher thermalstability than the ionomer bonding agents, such as Nafion®.

The stabilizer may include one or more surfactants and/or one or moresimple organic ligands. Electrode ink surfactants may include cationic,anionic, amphoteric, and/or non-ionic surfactants. For example, suitablesurfactants may include sodium dodecyl sulfate (SDS), sodium laurethsulfate (SLS), cetyltrimethylammonium bromide (CTAB), polyethyleneglycol (PEG) derivatives (e.g., narrow range ethoxylates),fluorosurfactants, combinations thereof, or the like. Suitable simpleorganic ligands may include thiols, amines, acids, phosphines, polyols,combinations thereof, or the like. In various embodiments, an electrodeink may include less than 2 wt % of the stabilizer, such as from 0.25 wt% to about 1 wt %, or about 0.5 wt %, based on a total weight of theelectrode ink.

Electrode inks may be formed using any suitable mixing process, such asultrasonication, magnetic stirring, ball milling, planetary milling,high pressure mixing, mechanical mixing, other mechanical mixingmethods, combinations thereof, or the like. In some embodiments, thesolids component may be dispersed in the carrier of an electrode inkusing magnetic stirring and ultrasonication.

The present inventors realized that alcohol-based solvents, such as IPA,may react with catalyst particles and/or polymerize hydrophobic polymerbonding agents, such as PTFE. Accordingly, in one embodiment, electrodeinks including alcohol-based or alcohol containing solvents may alsoinclude water, a combination of alcohol-based and non-alcohol basedsolvents and/or a bonding component (i.e., agent) that includes both anionomer (e.g., a hydrophilic ionomer) and a hydrophobic polymer bondingagent.

In various embodiments, multi-layer anodes may be formed using two ormore different anode inks. For example, to form the bilayer anode 420, afirst anode ink may be used to form the first anode catalyst layer 422,and a second anode ink may be used for form the second anode catalystlayer 424.

The solids component of the first anode ink may include first catalystparticles, a hydrophobic bonding agent, such as PTFE, and an ionomerbonding agent, such as a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer having a chemical formula C₇HF₁₃O₅S.C₂F₄ soldunder the brand name Nation®. The solids component of the first anodeink may also include an optional catalyst support component.

While not wishing to be bound to any particular theory, it is believedthat sintering of the first anode ink at a relatively high temperature(e.g., the polymer bonding agent sintering temperature) may result inthe loss of functional chains from the molecular backbone of the ionomerbonding agent (e.g., Nafion®) included in the first anode ink. As aresult, the Nafion® in the first anode catalyst layer 422 may be leftwith a backbone having a similar structure to the PTFE hydrophobicbonding agent. Thus, the different bonding agents in the first anodecatalyst layer would not have significantly different properties fromeach other after the sintering. Furthermore, adding the ionomer bondingagent to the first anode ink in addition to the hydrophobic polymerbonding agent, such as PTFE, allows the use of an alcohol-based oralcohol containing solvent without significantly polymerizing thehydrophobic bonding agent, such as PTFE, or reacting with the catalystparticles.

The solids component of the second anode ink may include second catalystparticles, an ionomer bonding agent, such as the sulfonatedtetrafluoroethylene based fluoropolymer-copolymer having a chemicalformula C₇HF₁₃O₅S.C₂F₄ sold under the brand name Nafion®, and anoptional catalyst support component. The solids component of the cathodeink may include third catalyst particles, an ionomer bonding agent, andan optional catalyst support component.

The first, second, and third catalyst particles may include catalystparticles as described above. For example, the first and second catalystparticles may include a Pt and Ru catalyst, and the third catalystparticles may include a Pt catalyst. However, the first and secondcatalyst particles may have different Pt:Ru atomic ratios. For example,the first catalyst particles may have a Pt:Ru atomic ratio ranging fromabout 60:40 to about 40:60, or about 50:50, and the second catalystparticles may have a Pt:Ru atomic ratio ranging from about 55:45 toabout 70:30, or about 66:33. In particular, the second catalystparticles may have a higher Pt:Ru atomic ratio than the first catalystparticles. Alternatively, the first, second and third catalyst particlesmay comprise different metals or alloys from each other. For example, atleast one of the first, second and third catalyst particles may comprisePt—Ru alloy catalyst particles, and at least one other one of the first,second and third catalyst particles may comprise Pt—Mo and/or Pt—Coalloy catalyst particles.

In some embodiments, the first and second anode inks, and the cathodeink may include a carrier comprising water, ethyl lactate (whichcontains OH groups bound to a saturated carbon atom and may beconsidered an alcohol-based or alcohol containing solvent), and PGMEA,for example. The amount of water and the stabilizer component in thecarrier may be set to disperse both Nafion® and PTFE in the carrier.

The MEA 400 may be formed using the anode and cathode inks. The inks maybe dispensed directly onto MEA substrates, such as onto the GDL or ontothe polymer membrane. Electrode inks may be dispensed using anytechnique that can produce a uniform layer of catalyst particles on theintended substrate. For example, suitable dispensing processes mayinclude brushing, rod coating, screen printing, ultrasonic spraying, airspraying, roll to roll coating, slot die coating, or the like. In otherembodiments, the inks may be dispensed on a separate substrate, driedand/or sintered to form a catalyst decal, which may be transferred to anMEA substrate. In some embodiments, MEAs may be manufactured using acombination of the above processes.

In one embodiment, the first anode ink may be deposited on the first GDL426, air-dried, and heat-treated (e.g., sintered) to form the firstanode catalyst layer 422. For example, the first anode ink may beair-dried at room temperature, for from about 10 minutes to about anhour, and then heated at a first temperature ranging from about 280° C.to about 420° C., such as from about 300° C. to about 400° C., fromabout 325° C. to about 375° C., or about 350° C., for a time periodranging from about 15 minutes to an hour, such as about 30 minutes. Thishigh temperature sintering may result in the loss of functional chainsfrom the molecular backbone of the ionomer bonding agent (e.g., Nafion®)included in the first anode ink. As a result, the Nafion® in the firstanode catalyst layer 422 may be left with a backbone having a similarstructure to the PTFE hydrophobic bonding agent. In various embodiments,the heating may be conducted in an inert atmosphere or in an activeatmosphere (e.g., a reducing or oxidizing atmosphere), in order toactivate the first catalyst particles.

The first anode ink may include the first catalyst particles, andionomer bonding agent (e.g. Nafion®), and a hydrophobic bonding agent(e.g., PTFE), and optionally a catalyst support, dispersed in a carrier.In some embodiments, the first anode ink may be dispensed using rodcoating. The carrier may comprise water, ethyl lactate, PGMEA, and/or anoptional stabilizer component, for example. The amounts of water, ethyllactate, PGMEA, and/or stabilizer in the carrier may be configured todisperse both Nafion® and PTFE in the carrier.

The second anode ink may then be deposited on the first anode catalystlayer 422, air-dried, and sintered to form the second anode catalystlayer 424. For example, the second anode ink may be air-dried at roomtemperature, for from about 10 minutes to about an hour, and then heatedat a second temperature ranging from about 50° C. to about 80° C., suchas from 55° C. to about 65° C., or about 60° C., for a time periodranging from about 8 hours to about 14 hours, such as about 12 hours.The second temperature is lower than the first temperature, but thesecond heating step is longer than the first heating step. Therelatively lower heating temperature of this step may preserve thestructure of the Nafion® bonding agent in the second anode catalystlayer 424, such that the ionomer does not result in the loss offunctional chains from the molecular backbone. In various embodiments,the sintering may be conducted in an inert atmosphere or in an activeatmosphere (e.g., a reducing or oxidizing atmosphere), in order toactivate the second catalyst particles.

The cathode ink may be applied to the GDL 436, air-dried, and sinteredto form the cathode 430. For example, the cathode ink may be air-driedat room temperature, for from about 10 minutes to about an hour, andthen heated at a temperature ranging from about 50° C. to about 80° C.,such as from 55° C. to about 65° C., or about 60° C., for a time periodranging from about 8 hours to about 14 hours, such as about 12 hours. Invarious embodiments, the heating may be conducted in an inert atmosphereor in an active atmosphere (e.g., a reducing or oxidizing atmosphere),in order to activate the third catalyst particles. The cathode ink maybe applied before, after, or during the application of the first and/orsecond anode inks.

In some embodiments, electrode layers that include an ionomer as abonding agent, such as the cathode 430 and/or the second anode catalystlayer 424, may be manufactured on separate supports. The second anodecatalyst layer 424 may be removed from the support and applied/bonded tothe first anode catalyst layer 422 by a decal lamination process, afterthe first anode catalyst layer 422 is bonded to the anode GDL 426 (orthe microporous layer 428 on the anode GDL 426, if present). In otherembodiments, the second anode catalyst layer 424 may be removed from thesupport and applied/bonded to the membrane 440, by a decal laminationprocess. In some embodiments, the cathode 430 may be removed from thesupport and applied/bonded to the cathode GDL 436 by a decal laminationprocess.

In other embodiments, the second anode catalyst layer 424 and/or thecathode 430 catalyst layer 432 may be removed from their respectivesupports and bonded to opposing sided of the membrane 440. The firstanode catalyst layer 422 may be bonded to the anode GDL 426, and thendisposed on the second anode catalyst layer 424, by disposing the anodeGDL 426 on the membrane 440, and the cathode GDL 436 may be disposed onthe cathode 430 catalyst layer 432.

In other embodiments, the anode 420 may be formed by depositing thesecond anode ink on the membrane 440, and then drying and sintering thesecond anode ink to form the second catalyst layer 424, which may bebonded the membrane 440. The first anode ink may be deposited on ananode side of the anode GDL 426 (or the microporous layer 428, ifpresent), and then dried and sintered to form the first catalyst layer422, which may be bonded to the second catalyst layer 424. The cathode430 catalyst layer 432 may be formed by depositing the cathode ink onthe cathode GDL 436 or a cathode side of the membrane 440.

In various embodiments, the cell 400 may include gaskets 450 disposed onopposing sides of the MEA 410. For example, the gaskets 450 may bedisposed in grooves formed in the flow field plates 402. The gaskets 450may be formed of PTFE, ethylene propylene diene monomer (EPDM) rubber,silicon, fluorinated-silicon, fluorine rubber, fluorinated carbon-basedsynthetic rubber, or the like. The gaskets 450 may be flat gaskets ormay be formed by dispensing a gasket material using a liquid injectionmolding process or the like.

In various embodiments, the MEA 410 may include one or more optionalsub-gaskets 452. The sub-gaskets 452 may be disposed on the membrane 440and may be configured to at least partially define an active area of themembrane 440. The sub-gaskets 452 may be single or multi-layeredstructures that are bonded to the membrane 440. For example, thesub-gaskets 452 may be mechanically bonded, thermally bonded, oradhesion bonded to the membrane 440 and/or the anode and cathode 420,430. The sub-gaskets 452 should be formed of a material that iscompatible with the gaskets 450 and membrane 440 under operatingconditions. For example, the sub-gaskets 452 may be formed of PTFE,polyamide, polyimide, polyphenylene sulfide, polyethylene napthalate,polysulfone, polysupersulfone, polyethylene terephthalate, or the like.The thickness of the sub-gaskets 452 may be determined in conjunctionwith the thicknesses of other elements of MEA 410, such as thethicknesses of the membrane 440, anode and cathode GDLs 426, 436, anode420, cathode 430, and gaskets 450.

In various embodiments, the MEA 410 may be assembled by disposing themembrane 440 between the anode GDL 426 and the cathode GDL 436, suchthat the cathode 430 contacts a first side (e.g., cathode side) of themembrane 440 and the second anode catalyst layer 424 contacts anopposing second side (e.g., anode side) of the membrane 440. Thesub-gaskets may be disposed on the membrane 440 during the assemblyprocess.

The assembled MEA 410 may then hot-pressed to bond the membrane 440,anode 420, cathode 430, anode and cathode GDL layers 426, 436, and/orsub-gaskets 452. The hot-pressing may be performed at a temperature andpressure sufficient to bond the layers of the MEA 410, withoutcompromising the characteristics of the membrane 440 and the anode andcathode GDLs 426, 436. For example, the MEA 410 may be hot-pressed at atemperature ranging from about 40° C. to about 160° C., such as fromabout 60° C. to about 140° C., and at a pressure ranging from about 15kg/cm² to about 75 kg/cm², such as from about 25 kg/cm² to about 60kg/cm². The hot-pressing may be performed for a duration ranging fromabout 30 seconds to about 15 minutes, such as from about 1 minute toabout 10 minutes. In some embodiments, the hot-pressing may beconfigured to convert the membrane 440 into a sodium form, in order toimprove membrane stability.

Pump Separators

FIG. 5 is a sectional view of a pump separator 150 including anelectrochemical cell stack 500, according to various embodiments of thepresent disclosure. Referring to FIG. 5, the stack 500 may be disposedinside of a housing of the pump separator 150 and may include MEAs 410separated by flow field plates 402, as shown in FIG. 4. In addition, thestack 500 may include end plates 408 disposed on MEAs 410 disposed onopposing ends of the stack 500. The end plates 408 may be similar to theflow field plates 402, except that the end plates 408 each include onlyan inlet flow field 404 or an outlet flow field 406.

A voltage or current source 502 may be electrically connected to thestack 500. In particular, the voltage or current source 502 may beelectrically connected to the end plates 408, and may be configured toapply a voltage potential or a current across the MEAs 410.

The fuel cell systems and components described herein may have otherembodiments and configurations, as desired. Other components may beadded if desired. Furthermore, it should be understood that any systemelement or method step described in any embodiment and/or illustrated inany figure herein may also be used in systems and/or methods of othersuitable embodiments described above, even if such use is not expresslydescribed.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

I/We claims:
 1. A method of forming membrane electrode assembly (MEA),comprising: dispensing a first anode ink comprising first catalystparticles, a hydrophobic polymer bonding agent and an ionomer bondingagent, dispersed in a first carrier; heat-treating the first anode inkto form a first anode catalyst layer of an anode; dispensing a secondanode ink on the first anode catalyst layer, the second anode inkcomprising second catalyst particles and an ionomer bonding agent,dispersed in a second carrier; heat-treating the second anode ink toform a second anode catalyst layer of the anode; dispensing a cathodeink; and heat-treating the cathode ink to form a cathode layer.
 2. Themethod of claim 1, wherein the first anode ink is dispensed on an anodegas diffusion layer (GDL), and the cathode ink is dispensed on a cathodeGDL.
 3. The method of claim 2, further comprising: assembling the MEA bydisposing the anode and the cathode on opposing sides of anionically-conductive proton exchange membrane; and hot-pressing the MEA.4. The method of claim 1, wherein: the first catalyst particles areconfigured to preferentially catalyze the oxidation of carbon monoxide;and the second catalyst particles are configured to preferentiallycatalyze the generation of hydrogen ions.
 5. The method of claim 1,wherein the hydrophobic polymer bonding agent comprisespolytetrafluorethylene (PTFE), and the ionomer bonding agent comprises asulfonated tetrafluoroethylene based fluoropolymer-copolymer.
 6. Themethod of claim 5, wherein: the heat-treating the first anode inkcomprises drying the first anode ink at room temperature, and heatingthe dried first anode ink at a temperature ranging from about 280° C. toabout 420° C., and the heat-treating the second anode ink comprisesdrying the second anode ink at room temperature, and heating the driedsecond anode ink at a temperature ranging from about 50° C. to about 80°C.
 7. The method of claim 6, wherein: the heating the dried first anodeink results in a loss of functional chains from a molecular backbone ofthe ionomer bonding agent; and the heating the dried second anode inkdoes not result in a loss of functional chains from a molecular backboneof the ionomer bonding agent.
 8. The method of claim 1, wherein thefirst carrier and the second carrier comprise water and an organicsolvent.
 9. The method of claim 8, wherein the organic solvent includesan alcohol-based or an alcohol containing solvent.
 10. The method ofclaim 8, wherein the organic solvent comprises glycerol, ethyleneglycol, propylene glycol, N-methyl-2-pyrrolidone (NMP), isopropylalcohol (IPA), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK),acetone, ethyl lactate, diglyme, propylene glycol monomethyl etheracetate (PGMEA), butyl acetate, heptanol, or a combination thereof. 11.The method of claim 10, wherein the organic solvent comprises a mixtureof ethyl lactate and PGMEA.
 12. The method of claim 1, wherein the firstcarrier and the second carrier further comprise a stabilizer.
 13. Themethod of claim 1, wherein: the first catalyst particles comprise afirst Pt—Ru alloy; and the second catalyst particles comprise a secondPt—Ru alloy having a higher Pt:Ru ratio than the first Pt—Ru alloy. 14.The method of claim 13, wherein: the first anode layer comprises thefirst catalyst particles at a loading ranging from about 0.8 mg/cm² toabout 1.2 mg/cm²; the second anode layer comprises the second catalystparticles at a loading ranging from about 0.5 mg/cm² to about 0.9mg/cm²; and the loading of the first catalyst particles is higher thanthe loading of the second catalyst particles.
 15. The method of claim 1,wherein the cathode ink comprises platinum catalyst particles and ahydrophobic bonding agent, dispersed in a carrier.
 16. A fuel cellsystem comprising: a fuel cell stack; and the MEA of made by the methodof claim 1 configured to remove hydrogen from fuel exhaust generated bythe fuel cell stack.
 17. An anode ink for forming an anode layer of acarbon monoxide (CO) tolerant membrane electrode assembly (MEA),comprising: catalyst particles comprising platinum or a platinum alloy;an ionomer binding agent; a hydrophobic polymer binding agent; at leasttwo solvents selected from glycerol, ethylene glycol, propylene glycol,N-methyl-2-pyrrolidone (NMP), isopropyl alcohol (IPA), methyl ethylketone (MEK), methyl isobutyl ketone (MIBK), acetone, ethyl lactate,diglyme, propylene glycol monomethyl ether acetate (PGMEA), butylacetate, or heptanol; and water.
 18. A membrane electrode assembly(MEA), comprising: an ionically-conductive proton exchange membrane; ananode contacting a first side of the membrane, the anode comprising: ananode gas diffusion layer (GDL), a first anode catalyst layer containingfirst catalyst particles, a hydrophobic polymer bonding agent, and afirst ionomer bonding agent that lacks functional chains on a molecularbackbone; and a second anode catalyst layer containing second catalystparticles and a second ionomer bonding agent that includes functionalchains on a molecular backbone; and a cathode contacting a second sideof the membrane and comprising third catalyst particles and a cathodeGDL.
 19. The MEA of claim 18, wherein: the first catalyst particles areconfigured to preferentially catalyze oxidation of CO; and the secondcatalyst particles are configured to preferentially catalyze generationof hydrogen ions.
 20. The MEA of claim 18, wherein the hydrophobicpolymer bonding agent comprises PTFE, and the first ionomer bondingagent comprises a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer that lacks the functional chains on themolecular backbone.